THERMAL RESISTANCE OF TWO STRAINS OF LISTERIA MONOCYTOGENES

Microbiology
THERMAL RESISTANCE OF TWO
STRAINS OF LISTERIA MONOCYTOGENES
AFSHAN TALAT*
ROQUYA SIDDIQI*
SUMMARY: The thermal resistance of two strains of Listeria monocytogenes, strain NCTC 7973
and NCTC 5105, was demonstrated at five different temperatures (52,57,62,67, 72°C) in pasteurized
and whole boiled milk. 3 ml portions of milk suspended at concentrations of ca. 2.23x155 cells ml-1
was heated at temperatures ranging from 52 to 72°C. The D-value of NCTC 7973 for fresh boiled
milk ranged from 1620 to 60 sec. In pasteurized milk the D-value was computed ranging from 1860
to 75 sec. Moreover the Z-value for the same strain was 14.2°C and 11.6°C for fresh boiled milk and
pasteurized milk respectively. The D-value of strain NCTC 5105 ranged from 1050 to 120 sec. for
fresh boiled milk whereas for pasteurized milk it ranged from 2100 to 69 sec. The Z-value of NCTC
5105 was 21.5°C and 13.8°C for fresh boiled milk and pasteurized milk respectively.
Key Words: Listeria monocytogenes, D-value, Z-value, strain.
INTRODUCTION
Listeria monocytogenes has recently been reported
Milk samples
as food borne pathogen in four food borne out breaks in
Different brands of pasteurized milk samples were
United States and Canada. Food associated with these
obtained from the local market, whereas fresh boiled milk
outbreaks were vegetables, pasteurized milk and soft
samples were obtained from home delivery service and boiled
cheese (8,14,19). Besides such outbreaks L. monocyto-
before use.
genes has frequently been isolated from uncooked meat
(7,10,16) milk (5,9,12,17) and soft cheese (5,15). The
Stock cultures
ability to survive and grow at refrigeration temperatures
Cultures of L. monocytogenes were maintained on TPA
(11) and tolerance to the preserving agents, sodium chlo-
(Tryptose phosphate agar) slopes with monthly transfers and
ride and sodium nitrite (20), makes Listeria of particular
stored at 4°C in screw caped bottles.
concern as a post contamination agent for refrigerated
food. The ability of organism to grow at refrigeration tem-
Thermal resistance of L. monocytogenes in fresh boiled milk
perature is not well documented. Consequently present
and pasteurized milk samples
study was under taken to determine the thermal resist-
To determine the thermal resistance of two strains of L.
ance of L. monocytogenes in whole fresh boiled milk and
monocytogenes 100 ml of milk sample was withdrawn asepti-
pasteurized milk between 52 and 72°C. It is expected that
cally in a sterile conical flask. The sample was then inoculated
studies like this would provide some knowledge about the
with 1 ml inoculum ca. 2.23x107 cells ml-1 and flask was shaken
role of listeric infections.
well for even distribution of organisms. Portions of 3 ml of inocu-
lated milk sample was dispensed in sterile test tube. Samples
MATERIALS AND METHODS
were heated in a water bath (Kottermann). After intervals tubes
Bacterial strains
were taken out from water bath and ten fold dilution of the
Two strains of L. monocytogenes NCTC 7973 serovar 1/2a
sample was made. Number of survivors was determine by SPC.
and NCTC 5150 serovar 3a were used in the present study.
D-value was calculated from the data plotted on semilog paper,
*From Department of Microbiology, University of Karachi, Karachi
whereas Z-value was calculated from the curve D-value verses
75270, Pakistan.
heating temperatures.
Journal of Islamic Academy of Sciences 3:4, 330-332, 1990
330

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THERMAL RESISTANCE OF L. MONOCYTOGENES
TALAT, SIDDIQI
RESULTS
gested that pasteurized milk used to make cheese was a
Results of thermal resistance of two strains of L.
possible source of an outbreak of listeriosis in 1979 (18).
monocytogenes NCTC 7973 and NCTC 5105 in fresh
Reports on thermal resistance of L. monocytogenes
boiled milk and commercially available pasteurized milk at
have produce conflicting results (1,2). The most compre-
temperatures between 52-72°C are shown as D-value in
hensive study on heat resistance of L. monocytogenes
Table 1, moreover Z-value of each strain for both types of
was reported by Bradshaw et al. in 1985. Their data
milk is also shown in the same table.
revealed that L. monocytogenes does not tolerate the
The D-value estimated for NCTC 5105 in case of
lowest legal pasteurization temperature. Whereas
fresh boiled milk ranged from 1050-120 seconds. The
Garayzabal et al. in 1986 isolated L. monocytogenes
observed Z-value was 21.5°C. Initially ca. 2.23x105 cells
from ca. 21% of milk samples pasteurized at 78°C for 15
were suspended in fresh boiled milk and heated at differ-
seconds in processing plant in Spain. This finding was
ent temperatures. Results obtained indicated a linear
supported by Doyle et al. (1987) who interpreted their
decrease in population density. D-value obtained for
results that L. monocytogenes is more thermotolerant
NCTC 7973 ranged from 1620-66 seconds. The
than most non spore forming bacterial pathogenes and
observed Z-value was 14.2°C. The D-value at 72°C was
may in some instance survive minimum pasteurization
66 seconds for NCTC 7973 whereas it was D-value at
temperature treatment.
72°C was 66 seconds for NCTC 7973 whereas it was
There is no information available about the presence
120°C for NCTC 5105 at the same temperature. The
of Listeria in fresh raw milk sued in our country. It is possi-
comparative study of thermal resistance of these two
ble that Listeria may be present in the milk because of
strains revealed that NCTC 5105 was more heat resistant
shedding from a symptomatic animals. Such occurrence
then NCTC 7973 in fresh boiled milk.
in raw milk has previously been noted in several countries
The D-value in pasteurized milk ranged from 2100-60
(18). Hence it seems reasonable to believe that fresh raw
seconds and 1860-75 seconds for NCTC 7973 and
milk of different animals used here may contain consider-
NCTC 5105 respectively. Whereas Z-value in pasteurized
able amount of Listeria which may cause a hazard at any
milk was 13.8°C for NCTC 5105 and 11.6°C for NCTC
moment. The same fresh milk contaminated with Listeria
7973. The D-value obtained at 72°C was 60 seconds
when undergoes pasteurization process and if the organ-
which is less than D-value obtained for NCTC 5105 and
ism survive the minimum pasteurization temperature as
NCTC 7973 in fresh boiled milk at the same temperature.
reported by Doyle et al. (6) then people using this type of
Data indicate that both tested strains of L. monocyto-
milk are really at risk. Data of present study on thermal
genes appeared to be more heat resistant in fresh boiled
resistance of L. monocytogenes demonstrated that the
milk as compared to pasteurized milk.
thermal resistance of NCTC 7973 appeared to be higher
in fresh boiled milk as compared to pasteurized milk. Sim-
DISCUSSION
ilar results were obtained for NCTC 5105. Thermal resist-
The role and importance of L. monocytogenes as an
ance of L. monocytogenes has also been observed by
agent of food borne disease is becoming increasingly
Bradshaw et al. in 1986, who used L. monocytogenes
apparent. Recent outbreaks of listeriosis have led to the
strain Scott A, using different raw and sterile milk products
implication of a variety of foods, particularly dairy prod-
found that L. monocytogenes showed resistance in sterile
ucts as source of the pathogen (8,13,19). It was sug-
milk products.
Table 1: D-Value estimates for two strains of L. monocytogenes.
D-Value (Seconds)
Fresh Boiled Milk
Pasteurized Milk
Temperature
NCTC 7973
NCTC 5105
NCTC 7973
NCTC 5105
1 52°C
1620
1050
1860
2100
2 57°C
900
630
1410
750
3 62°C
150
150
168
360
4 67°C
90
138
111
102
5 72°C
66
120
75
60
Z-Value °C
14.2°C
21.5°C
11.5°C
13.8°C
331
Journal of Islamic Academy of Sciences 3:4, 330-332, 1990

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THERMAL RESISTANCE OF L. MONOCYTOGENES
TALAT, SIDDIQI
We report that the D-value for NCTC 7973 ranged
8. Fleming DW, Cochi SL, Mac Donald KL, Brondum J,
from 1620 - 66 seconds in case of fresh boiled milk,
Hayes PS, Plikaytis BD, Holems MB, Audurier A, Broome CV,
whereas it was 1860 - 75 seconds in pasteurized milk.
Reingold AL : Pasteurized milk as vehicle of infection in an out-
NCTC 5105 showed a D-value range from 1050 to 120
break of listeriosis. New Engl J Med, 312:404-407, 1985.
seconds and from 2100 to 60 seconds for fresh boiled
9. Garayzabal JFF, Rodiguez LD, Boland JAV, Cancelo
and pasteurized milk respectively. The Z-value for NCTC
JLB, Fernandez GS : L. monocytogenes dans le lait pasteurize.
7973 in pasteurized milk was 11.6°C, whilst it was 14.2°C
Can J Microbiol, 32:149-150, 1986.
in fresh boiled milk. The Z-value for NCTC 5105 was
10. Gitter M : L. monocytogenes in oven ready poultry. Vet
13.8°C and 21.5°C, for pasteurized and fresh boiled milk
Rec, 99:336, 1976.
respectively. These findings supported the idea that
11. Gray ML, Killinger AH : L. monocytogenesis and listeric
higher thermal resistance of both the strains in fresh
infections. Bacteriol Rev, 30:309, 1966.
boiled milk as compared to pasteurized milk may be due
12. Heyes PS, Feeley JC, Graves LM, Ajello GW, Fleming
to some unknown growth promoting factors present in
DW : Isolation of L. monocytogenes from raw milk. Appl Env
fresh boiled milk. These factors presumably inactivated in
Microbiol, 51:438-440, 1986.
commercially available pasteurized milk because of pro-
13. Ho JL, Shands KN, Friedland G, Eckind P, Fraser DW :
long storage before use.
An outbreak of type 4b L. monocytogenes infection involving
Comparative account of thermal resistance of both the
patients from eight Boston hospitals. Arch Inter Med, 146:520-
strains of L. monocytogenes in pasteurized milk and fresh
524, 1986.
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was more than NCTC 7973 in both types of milk products.
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Journal of Islamic Academy of Sciences 3:4, 330-332, 1990
332

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Document Outline

• JIAS Volume 3, No 4
• Thermal Resistance of Two Strains of Listeria Monocytogenes
• Summary
• Introduction
• Materials and Methods
• Results
• Discussion
• References

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